This disclosure generally relates to steam turbines. More particularly, this disclosure relates to methods for assessing cracking of turbine rotors and other metal turbine components exposed to water, steam and condensate in the steam turbine.
Steam turbine power systems use a fluid medium such as water or another suitable chemical fluid with boiling points and latent heat values appropriate for the operational temperatures of the system. The fluid medium is generally heated in a separate heat source such as a boiler by using directed solar radiation, burning of fossil fuel, nuclear radiation or geothermal energy. The energy is transferred from the heat source to a turbine(s) in the form of high-pressure steam that powers the turbines. The steam turns a rotor in the turbine. The rotation of the turbine may be applied to drive an electromagnetic generator producing electricity.
A common type of steam turbine system includes a plurality of turbines in the form of a high-pressure turbine, an intermediate pressure turbine, and a low-pressure turbine. The turbines can be in a closed loop, which includes a steam generator for supplying steam to the high-pressure turbine and a condenser that receives the low-pressure turbine discharge. Water from the condenser is provided back to the heat source, e.g., steam generator, for reuse and is generally treated prior to reuse to remove impurities. The steam turbine extracts energy from the steam to power an electrical generator, which produces electrical power. Alternatively, low to medium pressure steam, after passing through the turbines, can be directed to an intermediate temperature steam distribution system, e.g., a heat exchanger, that delivers the steam to a desired industrial or commercial application such as is desired for combined heat and power applications.
As shown in FIG. 1, each turbine generally includes a fixed partition, e.g., nozzles, and a plurality of turbine buckets 10, e.g., blades, mounted on rotatable turbine wheels 12. The buckets are conventionally attached to the wheels by a dovetail 14. Dovetail attachment techniques between turbine buckets and turbine rotor wheels for steam turbines are well known in the art of steam turbines. The outer rim of the turbine wheel includes a tangential entry dovetail connector 16 having a circular ridges on opposite sides of the wheel to secure the buckets. The rim of the turbine wheel and bucket dovetails have generally complementary pine tree cross-sectional shapes.
Different types of dovetails may be employed. For example, finger-type and fir tree dovetails are used to secure the buckets and rotor wheel to one another. In a finger type of dovetail, the outer periphery of the rotor wheel has a plurality of axially spaced circumferentially extending stepped grooves for receiving complementary fingers on each of the bucket dovetails when the buckets are stacked about the rotor wheel. Pins are typically passed through registering openings of the dovetail fingers of each of the wheel and bucket dovetails to secure the buckets to the wheel. A fir tree dovetail connector includes a cutout for each bucket in the outer rim of the wheel. The cutouts may generally form a “V” and have ridges in their sides to secure a match dovetail in the bucket. A common difficulty with all types of dovetail configurations is that the dovetail connections between the buckets and wheels are highly stressed and, after years of operation, tend to wear out and crack.
Cracking of the various components in low-pressure turbines, such as at the dovetail connection, is believed to be related to a phenomena commonly referred to as stress corrosion cracking (SCC). Stress levels within the component can accelerate SCC. In particular, the stresses present in the hook fillet regions of typical dovetail configurations can accelerate SCC. Normally, these stresses are acceptable but with contaminated steam and age, cracks can initiate and, if left undetected, may grow to a depth that will cause failure of the wheel hooks. Moreover, the steam at the low-pressure end of the turbine, contaminated or otherwise, is at a lower temperature having been cooled during passage through the turbine. As a result, water condenses therefrom more readily and as a result, the steam at the low pressure end of the steam turbine is fairly saturated with water. Because of exposure to the steam, the transfer of energy by impact of the wet steam by itself on the turbine blades is greater at the low-pressure end of the turbine than that at the high-pressure end, resulting in greater stress applied to the turbine components.
The steam environment existing in the steam turbine considerably affects the rate of progress of SCC. As used herein, the term “steam environment” refers to an environment in which water droplets, water films, or capillary condensates exist. The reason for this is that chemical factors are involved in stress corrosion cracking so that stress corrosion cracking is promoted in certain specific temperature regions dependent on the relationship between the steam constituents and the chemical properties of the rotor material. Because of the mass and the rotational speed of a turbine, e.g., typically on the order of 3,600 revolutions per minute (rpm), significant damage to the turbine, its housing and surrounds, as well as injury to turbine operators, can occur should cracks develop in the wheel dovetail sufficiently to permit one or more of the buckets to fly off the rotor wheel. In extreme cases, all the hooks will fail and buckets will fly loose from the rotor. Long experience with bucket-to-wheel dovetail joints has generally indicated that the wheel hooks can crack through an SCC mechanism but that the bucket hooks typically do not crack through an SCC mechanism.
The duty cycle of a turbine is the operational schedule that states its operating steps and conditions, such as turbine inlet temperature, speed and output power; period at each power output level, and time between maintenance shutdowns.
The remaining life of a turbine dovetail is dependent on the amount of SCC and the rate at which it will progress over the life of the component and also the operating condition. The low-pressure (LP) sections of nuclear and fossil fuel driven steam turbines are especially susceptible to SCC. During periodic inspections, the amount of SCC is determined for each turbine rotor or for a representative sample of turbine rotor. Depending on the amount of SCC determined from the inspection, a decision is made as to whether to return the turbine to operation or to repair the turbine. This decision is typically made by a skilled technician or engineer who has inspected the cracks in the dovetails of the turbine buckets. Moreover, there is a need for such an analytical tool to assist in predicting the remaining life of cracked dovetails.